1. Field of the Invention
The invention relates to a catalyst protection device and catalyst protection method for an internal combustion engine.
2. Description of Related Art
Some internal combustion engines (engines) include an exhaust purification catalyst (hereinafter, simply referred to as catalyst where appropriate) for purifying exhaust gas. However, when the catalyst is excessively heated by high-temperature exhaust gas; or the like, the purification capacity of the catalyst may decrease. In order to keep the high purification capacity of the catalyst, a technique for cooling the catalyst by decreasing the temperature of exhaust gas with the use of the heat of vaporization of fuel through an increase in fuel injection amount may be used. The increase in the amount of injected fuel in this case is termed over temperature protection (OT) increase. The amount of injected fuel that is increased through the OT increase is termed OT increase value.
Japanese Patent Application Publication No. 2011-220214 (JP 2011-220214 A) describes a technique for preventing an overheat of a catalyst by calculating an OT increase value when a current temperature that is calculated on the basis of an operating state of an internal combustion engine exceeds an OT determination temperature. An excessive OT increase may lead to an increase, in emissions of hydrocarbons (HC) and carbon monoxide (CO). In addition, an excessive OT increase is disadvantageous in terms of fuel economy. Therefore, in JP 2011-220214 A, a correction coefficient is calculated, and fuel is injected on the basis of the OT increase value corrected by the correction coefficient. The OT determination temperature is incorporated in calculation of the correction coefficient.
However, in JP 2011-220214 A, the OT determination temperature is used in calculation of the correction coefficient, so the OT increase value is excessively reduced. As a result, there is a possibility that the catalyst is not sufficiently cooled.